1. Field of the Invention
The present invention relates to a sealing device having a plurality of lips.
2. Description of Related Art
A sealing device 100 (i.e. a so-called oil seal) has been used in order to prevent leakage of a sealing fluid in a driving force transmitting section or the like where the sealing fluid (e.g. a working fluid or a hydraulic fluid) is used to drive a unit. Referring to a cross-sectional view in FIG. 6, the sealing device 100 prevents fluid to be sealed from leaking via an annular gap 103 between an inner surface of an axial opening 101a of a housing 101 and an outer surface 102a of a rotary shaft 102 inserted into the axial opening 101a. The sealing device 100 is annular and includes: a reinforcing ring 105 having a substantially L-shaped cross section; a fitting part 106 made of an elastic rubber material and covering the reinforcing ring 105 as an integral part thereof; and a seal portion 107. The reinforcing ring 105 is provided with an axial part 105a extending along the housing 101, and a radial part 105b folded radially inward (at a non-sealed side O) at one end of the axial part 105a. 
The sealing section (or portion) 107 includes a main seal 107a which extends from the radial part 105b toward the sealed side M and is in contact with the outer surface 102a, a rubbing surface, of the shaft 102, and a dust seal 107b which extends toward the atmosphere (or air or non-sealed) side O and is in contact with the outer surface 102a. 
The main seal 107a is provided with first and second lips 107c and 107d along the axial of the sealing device 100 since a single lip cannot maintain stable contact with the shaft 102 which not only rotates but also reciprocates, or which may be entered into the axial opening 110a in an eccentric manner.
The main seal portion 107a can behave reliably because of the two lips 107c and 107d. 
In order to improve the sealing performance of the main seal 107a, the first lip 107c has ribs 107e for promoting a pumping effect. The ribs 107e maybe realized by forming spiral grooves or ridges, or forming slits or the like, depending upon effects to be accomplished. A spring 108 applies tension to the main seal 107a to urge the lips 107c and 107d toward the shaft 102.
In the foregoing related art, only the first lip 107c is provided with the ribs 107e. There has been a strong demand or improving the sealing performance in order to promote the pumping effect.
Generally speaking, the sealing performance of the lip is determined hydrodynamically on the basis of the tension applied to a rubbing surface by a lip tip, a lip angle formed by a non-sealed side slope of a sealing lip and the axial direction of the sealing device (e.g. a lip angle actually formed by the slope of the lips 107d, 107c and the axial direction of the sealing device 100, in a cutting plane passing through the axially center line of the sealing device 100), and properties of the material (surface roughness or the like) of the lips. The sealing device is usually designed by appropriately setting the foregoing parameters. Further, the pumping effect realized by the ribs also contributes to improvement of the sealing performance.
It is usually considered that the pumping effect is realized by varying distribution of contact pressure with an outer surface of a shaft, in the axial direction of the shaft, in an area in contact with the outer surface 102a, the rubbing surface, of the shaft 102 with which a lip tip comes into contact near the ribs (e.g. a gradient of pressure directed to the sealed side M from the non-sealed side O is made as small as possible in order to allow the sealing fluid to move with ease), and by introducing the leaked fluid from the air side to the rubbing surface via the ribs.
One of the most difficult problems to be avoided is that the lip tip may be worn out and changed in shape during the operation of the sealing device. Further, there is a problem that the pumping effect of the ribbed lip may change with progress of wearing of the ribs during use of the sealing device.
A lip that is ribbed to its tip can pump during its initial use in a particularly effective manner at the time there is a relative movement between the sealing device and shaft. However, such a lip is most extensively deformed or worn during initial use because of abrasion of the ribbed portion, and there is a large variance, with operation of the sealing device, in the balance between an amount of fluid leaking onto the rubbing surface and an amount of fluid returned by the pumping effect.
Therefore, there has been demand for a sealing device that can maintain its reliable sealing performance for a long period of time as long as the lips can cope with the eccentricity of the shaft it is attached and can demonstrate high pumping effect despite of wearing (or abrasion) of a ribbed lip at the initial stage of use.
The present invention has been contemplated in order to overcome the foregoing problems of the related art, and is intended to provide a sealing device which can maintain reliable sealing performance for a long period of time from the initial use.
According to the invention, there is provided a sealing device comprising: a fitting portion fixed on a first surface forming an annular gap with a second surface; and a sealing portion supported by a support provided in the sealing device and including a plurality of lips placed axially whose tips come into slidingly contact with the second surface. In the sealing device, the annular gap is sealed by the sealing device for the purpose of isolating a sealed side and a non-sealed side, and ribs for pumping fluid to be sealed are formed on each slope in the non-sealed side of the plurality of the seal lips.
This configuration is effective in pumping back the fluid to be sealed toward the sealed side, improving the sealing performance.
At least one of the lips is preferably provided with a non-ribbed portion between its lip tip and the sealed side end of ribs which are placed on the non-sealed side slope of the lip. Namely the tips of the ribs are not connected to the tip of the lip.
The non-ribbed portion around radially inner peripheral portion of the lip tip suppresses leakage of the fluid to be sealed even at the time there is no relative sliding movement between the shaft and the sealing lips during the initial use of the sealing device.
It is preferable that at least one of the lips nearer to the sealed side than the lip having a non-ribbed portion is provided at its tips with an annular or band-like contact area which has a lip angle that is smaller than a lip angle formed by surface of the sloped non-sealed side and the axial direction of the sealing device. The band-like contact area contacts circumferencially with the second surface. The contact area functions identically to the lip tips which have been worn out after the initial use, and is effective in assuring reliable sealing performance and promoting pumping fluid to be sealed back to the sealed side when the contact area is in contact with the second surface, or the rubbing surface.
The band-like contact area, which is faced to the non-sealed side, is more preferably provided with ribs in order to further promote pumping back the sealing fluid.
The ribs on the contact effectively pump back fluid to be sealed by the ribs from the beginning of its use.
After initial use and ribs on the contact area disappeared due to wear, pumping effect can be secured because ribs on the second lip appear due to abrasion of the second lip, exerting pumping effect.
Therefore, the sealing device can not only prevent the leakage of the sealing fluid during initial use but also assure a reliable pumping operation during use.
It is preferable that a lip angle of the band-like area is smaller than that of the lip having the band-like area. The lip angle is defined as an angle between a slope of non-sealed side and the axial direction of the sealing device.
The ribs are preferably realized by spiral ridges projecting from the slopes of each lips. Lips are ribbed on the surface of molds by cutting or stamping using hard objects. The molds can therefore be easily manufactured at a reduced cost.
It is preferable that the non-ribbed portion has a width between 0.1 to 0.5 mm from the lip tip.